HDFS-17879. Premature error handling accesses uninitialized dnInfo.

[balodesecurity]

Description of PR

JIRA: https://issues.apache.org/jira/browse/HDFS-17879

Problem

In DFSStripedInputStream.createBlockReader(), dnInfo is initialized with all-null fields before the retry loop:

DFSInputStream.DNAddrPair dnInfo =
    new DFSInputStream.DNAddrPair(null, null, null, null);

It is then populated by getBestNodeDNAddrPair() — but only after refreshLocatedBlock() is called first. If refreshLocatedBlock() throws an IOException, the catch block immediately accesses dnInfo.addr (still null), causing a NullPointerException that masks the original error:

NullPointerException at DFSStripedInputStream.java:267  (dnInfo.addr)
  caused by: IOException from refreshLocatedBlock()

Fix

Add a null check on dnInfo.addr at the top of the catch block. If null, the failure occurred before any DataNode was identified, so we rethrow the original IOException directly — preserving the real error for the caller.

if (dnInfo.addr == null) {
  throw e;
}

Testing

Added TestDFSStripedInputStream#testCreateBlockReaderExceptionBeforeDnInfoInit which uses Mockito to make refreshLocatedBlock() throw an IOException, and verifies:

• Before fix: NullPointerException is thrown (masked error)
• After fix: original IOException is propagated cleanly

For code changes:

• Does the title or this PR starts with the corresponding JIRA issue id (e.g. 'HADOOP-17799. Your PR title ...')?
• Object storage: have the integration tests been executed and the endpoint declared according to the connector-specific documentation?
• If adding new dependencies to the code, are these dependencies licensed in a way that is compatible for inclusion under ASF 2.0?
• If applicable, have you updated the LICENSE, LICENSE-binary, NOTICE-binary files?